1. Field of the Invention
The present invention relates to a heating device.
2. Description of Related Art
In a step for manufacturing a semiconductor device, a wafer is heated for forming an oxide film on the wafer by a semiconductor manufacture apparatus. One of heating devices in this semiconductor manufacture apparatus for heating a wafer is structured so that a resistance heating element is embedded in a disk-like ceramics base on which a wafer as a to-be-heated object is placed and is heated. This heating device is advantageously adapted not only to a film formation apparatus used for a semiconductor manufacture process but also to a surface processing apparatus such as a dry etching apparatus for etching the surface of a wafer.
When a to-be-processed object is heated by a heating device, the neighborhood of the to-be-processed object set on a heating surface of a ceramics base may be provided to have plasma atmosphere as in a film formation processing by plasma CVD or a plasma etching processing. One heating device is structured so that a high-frequency electrode for causing this plasma atmosphere is provided in the vicinity of a heating surface of a ceramics base so that the electrode is substantially in parallel with the heating surface. In the heating device as described above, a back face opposite to the heating surface of the ceramics base includes a hole for supplying power to this high-frequency electrode provided so as to face the high-frequency electrode. Power from outside is supplied to the heating device through power feeding material that is inserted to the hole so that the power feeding material is connected to the high-frequency electrode itself exposed at the bottom of the hole or a conductive member fixed to the high-frequency electrode.
With regards to the heating device as described above, a heating device has been disclosed in which a mesh-like high frequency electrode is embedded in ceramics base material of aluminum nitride and a high-frequency electrode exposed at a hole of this ceramics base material is joined to a Ni-made rod as a power feeding material by brazing material via terminals (Japanese Patent Unexamined Publication No. H08-277173).
Another heating device has been disclosed in which a Mo-made high frequency electrode having a mesh-like shape and a Ni-made rod sandwich kovar that has a thermal expansion coefficient in the middle of thermal expansion coefficients of these members and the Mo, kovar, and Ni members are joined by brazing material. Still another heating device has been disclosed in which a Mo-made high frequency electrode having a mesh-like shape is not directly joined to kovar material but the Mo-made high frequency electrode having a mesh-like shape is sintered together with a Mo bulk material having a diameter of 3 mm and a thickness of 2 mm to join this Mo bulk material with kovar material (Japanese Patent Unexamined Publication No. 2002-134590).
A mesh for this high-frequency electrode is a sheet obtained by knitting a thin metal wire and is mainly made of Mo. A high-frequency electrode using this mesh is shaped to have a flat surface embedded in parallel with a ceramics base heating surface in order to uniformize the distribution of plasma over a to-be-heated object. Thus, a mesh-like high frequency electrode that is at a part opposed to a hole formed in the ceramics base and that is joined by a terminal by gold solder also has a flat surface. A region between the mesh-like high frequency electrode in the ceramics base and the heating surface of the ceramics base is made of ceramics material such as aluminum nitride and functions as a dielectric material layer and has a thickness of about 1 mm in order to provide a uniform plasma distribution. A wafer is placed on a surface at this dielectric material layer (i.e., a heating surface).
The above-described structure of a high-frequency electrode is commonly used for a heater, an electrostatic chuck, and a susceptor.
An aluminum nitride-made ceramics base in which a mesh-like high frequency electrode is embedded has a thickness of about 5 to 25 mm. This thickness includes a thickness of about 1 mm of the aluminum nitride layer as the dielectric material layer between a high-frequency electrode and a heating surface as described above. A ceramics base is structured so that the dielectric material layer at a part opposed to the hole inserted with a power feeding material is weakest and has a low strength.
This has caused a risk where this thin dielectric material layer or an insulating material layer may have a crack when a region as a dielectric material layer between this high-frequency electrode and the heating surface that is opposed to the hole inserted with the power feeding material is subjected to a thrust for inserting the Ni rod made of this power feeding material to the hole or when this Ni rod is expands or contracts.
This will be described in detail. The Ni rod is attached to the ceramics base by inserting a connector formed at the tip end of the Ni rod to the hole of the ceramics base. Thus, the thrust force applied to the Ni rod and the connector is transmitted to a soldered section as a terminal of the high-frequency electrode to push up the thin dielectric material layer. This has caused a risk where this thrust may cause a crack. Furthermore, when the connector formed at the tip end of the Ni rod is joined and fixed to a joint section in the high-frequency electrode (i.e., the terminal section of the high-frequency electrode), the Ni rod repeats thermal expansion and contraction in accordance with a temperature increase and cooling of the ceramics base. This applies a cyclic stress to the thin dielectric material layer. This repeated stress also causes a risk of a crack.
The heating surface at the dielectric material layer of the ceramics base is exposed to highly-corrosive gas atmosphere including high-frequency plasma or fluorine when a to-be-heated object is processed. Thus, the condition of the heating surface gradually deteriorates after the use for a long period of time. This gradual deterioration causes increased cracks even when a load that is caused by the above-described thrust or repeated stress and that is applied to the thin dielectric material layer is fixed. This has caused a risk of a shorter life after the use for a long period of time. Furthermore, a space that has a contact with the upper side of the terminal section of the high-frequency electrode (dielectric material layer-side) in the ceramics base generally has a negative pressure that is a pressure in a chamber in which a to-be-heated object is heated, and a space having a contact with the Ni rod-side region is frequently blocked from the interior of the chamber to have an atmosphere pressure. The difference of these pressures has encouraged the generation and growth of the crack in the dielectric material layer.
Once such a crack is caused, the crack grows due to the presence of the above described pressure difference, plasma or corrosive environment, and the grown crack penetrates the dielectric material layer, causing the heater, the electrostatic chuck, or the susceptor to be nonusable.
In view of the above, it is an objective of the present invention to provide a heating device that advantageously suppresses the crack caused in the vicinity of the terminal section of the high-frequency electrode to provide a high reliability and a long life.